222 3-D textile reinforcements in composite materials 3-iagid 7.4 Solid braid fabrication and geometry. woo'ssaudmau'peaupoom//:dny Aq paiaanad WV IZ:IE:ZI I 1OZ 'ZZ Aenur 'Aupines CARR【E @】 CORE YARM ROTOR- 一F0:47tH 7.5 Method of advanced 3-D solid braiding (compliments of Toyoda Automatic Loom Works,Ltd)
222 3-D textile reinforcements in composite materials 7.4 Solid braid fabrication and geometry. 7.5 Method of advanced 3-D solid braiding (compliments of Toyoda Automatic Loom Works, Ltd). RIC7 7/10/99 8:21 PM Page 222 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9
Braided structures 223 step zero step one step two step three step four path of carrier"a" 巧巧巧巧西 7.6 The Cartesian braiding process. other.The next step involves the alternate shifting of the columns (or rows) a prescribed distance.The third and fourth steps are simply the reverse shifting sequence of the first and second steps,respectively.A complete set of four steps is called a machine cycle (Fig.7.6).It should be noted that after one machine cycle the rows and columns have returned to their original positions.The braid pattern shown is of the 1 x 1 variety,so-called iannun because the relation between the shifting distance of rows and columns is one-to-one.Braid patterns involving multiple steps are possible but they require different machine bed configurations and specialized machines.This unique'multi-step'braiding technique is what renders Cartesian braiding a versatile process.Track and column braiders of the type depicted in Fig.7.6 may be used to fabricate preforms of rectangular cross-section such as T- beam,I-beam and box beam if each column and row may be independently displaced.Cartesian braided composites offer excellent shear resistance and quasi-isotropic elastic behavior due to their symmetric,intertwined structure.However,the lack of unidirectional reinforcement results in low stiffness and strength,and high Poisson effect.To help eliminate this,some advanced machines allow for axial yarns to be fed into the structure during fabrication. 7.3.2 Braid architecture,yarn grouping and shapes If one allows for multiple steps in a machine cycle,independent displace- ment of tracks and columns,and non-braider yarn insertion,the Cartesian braiding process is capable of producing a variety of yarn architectures, hybrids and structures.Consider the eight-step braid cycle shown in Fig.7.7, which also shows the phenomenon of yarn grouping. Yarn groups are sets of yarn tows that travel the same path.A multistep braiding process may have multiple yarn groups and a varying number of
other. The next step involves the alternate shifting of the columns (or rows) a prescribed distance. The third and fourth steps are simply the reverse shifting sequence of the first and second steps, respectively. A complete set of four steps is called a machine cycle (Fig. 7.6). It should be noted that after one machine cycle the rows and columns have returned to their original positions. The braid pattern shown is of the 1 ¥ 1 variety, so-called because the relation between the shifting distance of rows and columns is one-to-one. Braid patterns involving multiple steps are possible but they require different machine bed configurations and specialized machines.This unique ‘multi-step’ braiding technique is what renders Cartesian braiding a versatile process. Track and column braiders of the type depicted in Fig. 7.6 may be used to fabricate preforms of rectangular cross-section such as Tbeam, I-beam and box beam if each column and row may be independently displaced. Cartesian braided composites offer excellent shear resistance and quasi-isotropic elastic behavior due to their symmetric, intertwined structure. However, the lack of unidirectional reinforcement results in low stiffness and strength, and high Poisson effect. To help eliminate this, some advanced machines allow for axial yarns to be fed into the structure during fabrication. 7.3.2 Braid architecture, yarn grouping and shapes If one allows for multiple steps in a machine cycle, independent displacement of tracks and columns, and non-braider yarn insertion, the Cartesian braiding process is capable of producing a variety of yarn architectures, hybrids and structures. Consider the eight-step braid cycle shown in Fig. 7.7, which also shows the phenomenon of yarn grouping. Yarn groups are sets of yarn tows that travel the same path. A multistep braiding process may have multiple yarn groups and a varying number of Braided structures 223 7.6 The Cartesian braiding process. RIC7 7/10/99 8:21 PM Page 223 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9
224 3-D textile reinforcements in composite materials (steps 1 2) (steps 3&4) (steps 5&6) (steps 7&8) step 1 step 2 step 3 step 4 Architecture b 8 step 5 step 6 step 7 step 8 Group "a has 5 yarns Group"b" has 9 yarns cccb c Group "c" has 9 yarns dd ccc Group "d"has 5 yarns Eight-step 1x1 cycle d d Yarn grouping 7.7 Sample multiple step cycle showing idealized architecture (repeat geometry)and yarn grouping. yarns per group.It is possible to tailor the location of the yarn groups within the preform cross-section.That is to say,the braid cycle (i.e.shifting 9 sequence of tracks and columns)that will yield the desired grouping of yarns may be determined and different fibrous material utilized for the tows that make up a given group.In this way,unique hybrid composite materials may be formed which benefit both from the 3-D integrated nature of the 分 braid and from the hybrid effect and select yarn placement. The existence of yarn groups implies that sets of yarns trace the same path on the machine bed.After one complete machine cycle,each yarn in a group has moved to its leading yarn's location.This in turn implies that the braid geometry produced during one machine cycle(repeat)is the repeating geometry for the entire structure.That is to say,a cross-sectional slab of preform with the length produced during one repeat may be 'stacked-up'on top of one another to reproduce the entire preform(Fig. 7.7).It is possible,within Cartesian braiding process limits,to specify this braid architecture and determine the braid cycle which will yield it.It may be seen that knowledge of this repeat braid geometry is essential for future prediction of braided composite properties. One way of producing a braided preform with a complex cross-sectional shape is through implementation of the universal method (UM)of braid- ing [10].The basic concept behind the UM is to cut the complex cross- section of the preform into finite rectangular elements and then to braid these elements in groups.Since any shape may be estimated through a suit- able number of rectangular elements,the UM provides a plausible means
yarns per group. It is possible to tailor the location of the yarn groups within the preform cross-section. That is to say, the braid cycle (i.e. shifting sequence of tracks and columns) that will yield the desired grouping of yarns may be determined and different fibrous material utilized for the tows that make up a given group. In this way, unique hybrid composite materials may be formed which benefit both from the 3-D integrated nature of the braid and from the hybrid effect and select yarn placement. The existence of yarn groups implies that sets of yarns trace the same path on the machine bed. After one complete machine cycle, each yarn in a group has moved to its leading yarn’s location. This in turn implies that the braid geometry produced during one machine cycle (repeat) is the repeating geometry for the entire structure. That is to say, a cross-sectional slab of preform with the length produced during one repeat may be ‘stacked-up’ on top of one another to reproduce the entire preform (Fig. 7.7). It is possible, within Cartesian braiding process limits, to specify this braid architecture and determine the braid cycle which will yield it. It may be seen that knowledge of this repeat braid geometry is essential for future prediction of braided composite properties. One way of producing a braided preform with a complex cross-sectional shape is through implementation of the universal method (UM) of braiding [10]. The basic concept behind the UM is to cut the complex crosssection of the preform into finite rectangular elements and then to braid these elements in groups. Since any shape may be estimated through a suitable number of rectangular elements, the UM provides a plausible means 224 3-D textile reinforcements in composite materials 7.7 Sample multiple step cycle showing idealized architecture (repeat geometry) and yarn grouping. RIC7 7/10/99 8:21 PM Page 224 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9